(1) Field of the Invention
This invention relates to improved means for selectively removing precursors of nitrogen oxides from tobacco without substantial removal of potassium by means of electrodialysis. In accordance with the invention, selective denitration is effected in an electrodialysis stack wherein an acid solution circulates through certain cells in the stack. By means of the stack arrangement of the invention, nitrate removal can be selectively effected without use of bipolar membranes, the availability and construction of which may present problems in operating and commercializing a process for selective denitration of tobacco by means of electrodialysis.
(2) Description of the Prior Art
Tobacco contains a number of nitrogen containing substances which during the burning of the tobacco yield various components in the smoke. Removal of some of these smoke components, such as the oxides of nitrogen, is considered desirable.
Nitrate salts, such as potassium, calcium and magnesium nitrates, are a major class of nitrogenous substances which are precursors for nitrogen oxides, especially nitric oxide. These nitrate salts are normally found in great abundance in burley tobacco stems and strip and to a lesser extent in flue-cured tobacco stems and in reconstituted tobaccos which utilize these components. Attempts have been made to reduce or remove the nitrate from these tobaccos to bring about a significant reduction in the oxides of nitrogen delivered in their smoke. Among the techniques which have been employed to this end are extraction methods whereby the nitrates are removed from the tobacco material.
In accordance with extraction techniques, tobacco materials are generally contacted with water. In this manner, an extract containing the tobacco solubles including the nitrates, is formed. The extract is collected and may be discarded or may be treated to remove the nitrates. The denitrated extract may thereupon be reapplied to the fibrous insoluble tobacco material from which it was originally removed.
Although extract treatment methods seek to minimize the removal of materials other than nitrates from the tobacco and thereby avoid affecting the subjective characteristics of the tobacco or its filling capacity, burn qualities and the like, other materials are in fact removed by such methods. For example, the nitrates are commonly removed as potassium salts. Specifically, U.S. Pat. Nos. 4,131,118 and 4,131,117 describe a denitration process wherein potassium nitrate is crystallized from an aqueous tobacco extract followed by reapplication of the denitrated extract to the tobacco. In U.S. Pat. No. 3,847,164 denitration is effected by means of ion-retardation resins which retard ionic material, specifically potassium nitrate, in tobacco extract, while non-ionic constituents pass unaffected. Thus, these methods remove not only nitrate ions, but also potassium ions.
In addition to denitration, extraction processes are employed where removal of other tobacco components is desired. For example, U.S. Pat. No. 3,616,801 describes a process for improving the tobacco burn properties, smoke flavor and ash by controlling the ion content of the tobacco. In accordance with the process therein disclosed the proportion of metallic ions in an aqueous tobacco extract is adjusted, followed by reapplication of the treated extract to the tobacco. Among the treatments suggested for adjusting the metal ion content are ion exchange and membrane electrodialysis. Removal of potassium ions and their replacement with ammonium, hydrogen, calcium or magnesium ions are particularly desirable in the practice of this process. Levels of other ions including nitrate may also be adjusted to alter the tobacco properties. In Example 6, over 50% of both nitrate and potassium ions were removed by means of electrodialysis.
In parent application Ser. No. 127,386, now U.S. Pat. No. 4,301,817, a denitration method is described wherein nitrate is removed from tobacco extract while the potassium ion level is maintained substantially intact. By such selective denitration a proportionately greater reduction in delivery of nitrogen oxides in tobacco smoke relative to degree of nitrate removal is achieved than when the potassium ions are also removed. Electrodialysis is one means described for effecting selective denitration. In the described process, extract is circulated through those cells of an electrodialysis unit having an anion permeable membrane toward the anode and a bipolar membrane toward the cathode while brine is circulated through alternate cells having the reverse membrane set-up.
Bipolar membranes formed by uniting anion and cation impermeable membranes often exhibit nonuniform joinder resulting in a very resistant bipolar membrane. It has now been discovered that the use of bipolar membranes can be avoided without reducing the efficacy of the previously described process. Such avoidance is possible by separation of the brine-extract cell pairs with an acid solution confined between an anion permeable and a cation permeable membrane rather than with a bipolar membrane. The acid solution further serves to neutralize the denitrated tobacco extract and in some cases the brine, thereby preventing pH rises and resultant precipitation of salts of multivalent cations in the membranes.